The presently disclosed subject matter relates to a TWA measuring apparatus and TWA measuring method which can measure the presence of TWA (T-Wave Alternans).
TWA appears at onset of illness such as QT prolongation syndrome, variant angina, acute myocardial ischemia, electrolyte abnormality, paroxysmal tachycardia, bradycardia, or pericardial fluid accumulation. TWA is a phenomenon in which the amplitude and polarity of the T wave appearing in an electrocardiogram are alternately changed, and an index effective to predict sudden cardiac death. TWA is not a phenomenon which can be always observed with the naked eye, and therefore its application in clinics is limited.
From the 1980s, consequently, techniques for enabling minute TWA (Microvolt TWA: MTWA) to be measured by a computer have been developed.
Examples of currently proposed techniques for measuring TWA are a measurement technique based on the MMA (Modified Moving Average) method of General Electric (GE) Company, and a measurement technique based on the periodogram of Cambridge Heart (CH), Inc., which are disclosed in U.S. Pat. No. 6,668,189 and U.S. Pat. No. 5,935,082, respectively.
The measurement technique of GE Company is directed to a method of analyzing a time waveform in a time region, and is said to have resistance to noises. However, the technique does not have a long history as a measurement technique, and it is required to watch its clinical effect.
By contrast, the measurement technique of CH Inc. which is a technique in a frequency region has been used from the 1980s, and hence its effectiveness in clinics has been proved. Today, therefore, it is considered that the measurement technique based on the periodogram of CH Inc. is clinically more useful than that based on the MMA method of GE Company.
With respect to the measurement technique based on the periodogram of CH Inc., after its announcement, various techniques for performing new processes, such as a technique of measurement electrodes are added, and still now the added latest techniques have been used.
The technique for measuring TWA based on the periodogram disclosed in U.S. Pat. No. 5,935,082 is based on the assumption that various conditions are satisfied, such as the heart rate must be kept constant in an exercise test, and the heart rate in an abnormal case such as extrasystole must be suppressed to a specified value or smaller.
Depending on the subject, however, there is a case where the heart rate cannot be kept constant in an exercise test. There is a further case where, even when the heart rate can be kept substantially constant, noises may enter in several beats. Moreover, the subject cannot suppress the heart rate in an abnormal case such as extrasystole to a specified value or smaller.
In the technique for measuring TWA disclosed in U.S. Pat. No. 5,935,082, even when measurement of TWA is tried to be performed without satisfying the above conditions, data collection for measuring TWA is not started. Even in the case where data collection is started, when the above conditions become unsatisfied in the middle of the measurement, the data collection is reset.
Furthermore, since the measurement is performed while satisfying the above conditions, the inspection time tends to be prolonged, and hence an extra burden is imposed on the measuring person and the subject. The TWA measurement is not ended unless the above conditions are satisfied. In the case where time is limited, therefore, the TWA measurement cannot be completed.